Extension Educator, Horticulture
The winter solstice, December 22, passes virtually unnoticed by most people today. For ancient agrarians, SAD support groups and our plants; however, the day with the shortest daylight of the year is more than just a reason to turn on the lights.
The winter solstice marks when the sun is at its farthest distance from the equator and the northern hemisphere. After December 22 periods of daylight gradually get longer until my favorite "garden until you drop" day, June 21, the longest daylight of the year.
Winter solstice has been celebrated for thousands of years by cultures throughout the world. It was a time to celebrate light. Ancient Romans celebrated Saturnalia starting on December 17. The seven day festival was named in honor of Saturn, the god of agriculture.
As far as we know, plants don't have any wild parties to mark the winter solstice; however, they do respond to the length of daylight or more accurately the length of darkness. The response of plants to the relative length of day and night is called photoperiodism. Plants actually "measure" the duration of dark and light with the help of a light sensitive pigment called phytochrome.
Photoperiodism influences many activities in plants including growth, seed germination, flowering, fruit development, and the onset of winter dormancy. Some plants need a specific ratio of day to night length to initiate flowering. The time required can vary among plant species and plant varieties.
It's really all about the dark. Short day/long night plants flower when night length exceeds a critical dark period. Interruption of dark, even briefly, will keep short day plants from blooming. In plants referred to as short day, their flower bud formation begins when daylight is relatively short about 10 to 12 hours. A few common plants that require short days/ long nights to bloom include dahlia, gardenia, poinsettia, chrysanthemum, kalanchoe, celosia, hyacinth bean vine and Spanish flag.
Long day/short night plants initiate flower bud formation once they are exposed to relatively long days and short nights, about fourteen hours or more of daylight. In some long day/short night plants a brief light interruption at night helps them to bloom.
Some of our common vegetables are long day plants such as beet, radish, lettuce, spinach and potatoes. For northern variety onions long days are needed for the bulb to form. Other plants that require long days/short nights to flower include aster, coneflower, most sunflowers, hosta, strawflower, dill, bachelor's button, fuchsia, gazania, sweet pea, flax, lobelia, monkey flower, love-in-a-mist, some petunias, black-eyed Susan, and daisy.
Various plants such as tulips, tomatoes and many trees don't care about day length to flower and are called "day neutral" plants. Of course all this plant response may change depending on temperature, humidity and plant maturity.
So what does all this mean to gardeners? Growers have to manipulate light to get some plants to bloom or not bloom at the appropriate time. Poinsettia flowering can be disrupted with lights from parking lots, so plants may have to be covered every night to get them to bloom this time of year. To coax long day plants such as Easter lilies into bloom in the spring, the daylight is extended and night length is shortened with supplemental lighting.
Short day garden plants such as hyacinth bean and Spanish flag vines may not flower well when planted near outdoor lighting. Some vegetable plants that we don't want to flower may need to be planted later or earlier in the year to get the correct response. For example, long day plants lettuce and spinach produce bountiful leaves under the short days of spring and fall, but quickly flower during the long days of summer. If a plant isn't flowering well, include its photoperiod requirement on the list of suspects.